1. Field of the Invention
The present invention relates to a poling method for a piezoelectric film and a manufacturing method for a piezoelectric element structure that includes a process of poling a piezoelectric film by the poling method.
2. Description of the Related Art
A piezoelectric device which includes a piezoelectric film having piezoelectricity, in which the film stretches or contracts according to the applied electric field strength, and an electrode for applying an electric field to the film is used as an actuator mounted on an inkjet recording head and the like.
As for piezoelectric materials, composite oxides having a perovskite structure, such as lead zirconate titanate (Pb(Zr, Ti)O3, hereinafter, “PZT”) and the like are known. Such a material is a ferromagnetic substance having spontaneous polarization property in the absence of electric field. It is known that PZT doped with any of various donor ions having a higher valence number than that of the ion to be substituted has improved properties in ferroelectric performance and the like and a reduced Curie temperature in comparison with pure PZT. As donor ions for substituting A-site Pb2+, various types of lanthanoid cations, such as Bi3+, La3+, and the like are known, and as donor ions for substituting B-site Zr4+ and/or Ti4+, V5+, Nb5+, Ta5+, Sb5+, Mo6+, W6+, and the like are known.
In order to use a piezoelectric body as a piezoelectric element, it is necessary to pole the piezoelectric body to orient the spontaneous polarization in a predetermined direction in advance. As for the poling method, it is common to form an electrode on each of the upper and lower surfaces of the piezoelectric body and to apply an electric field between the electrodes under a temperature condition of around 100° C.
Japanese Unexamined Patent Publication No. 5(1993)-160462 describes a polarization method that saturation polarizes a piezoelectric body in one direction and reversely polarizes the body by applying an electric field in a temperature from 0 to −50° C., thereby obtaining a desired polarizability value lower than that of the initial saturation polarization. Here, as the method for saturation poling the piezoelectric body, Japanese Unexamined Patent Publication No. 5 (1993) -160462 more specifically describes a method in which the polarization is performed on the piezoelectric body of 0.2 mm thick in an insulating oil under a condition of 600V at 80° C. for 30 minutes.
Recently, the thickness of piezoelectric bodies of piezoelectric elements has been reduced with improved piezoelectric properties, and now a piezoelectric element having a piezoelectric film of less than 100 μm thick is proposed.
Piezoelectric films may be formed by various types of film forming methods, such as sputtering, deposition, sol-gel process, laser abrasion, metal organic deposition (MOD), metal organic chemical vapor deposition (MOCVD), and the like.
It is known that, among these methods, when a piezoelectric film is formed by sputtering, the piezoelectric film has spontaneous polarization oriented in a direction from the side of the substrate toward the side of the film surface just after formed without poling.
In order to use a piezoelectric film formed on a substrate by sputtering as a piezoelectric element, the film is formed on the substrate with a lower electrode layer formed thereon in advance, i.e., on the lower electrode, and an upper electrode layer is formed on the piezoelectric film. When a piezoelectric element structure with a piezoelectric film having spontaneous polarization oriented in the direction from the substrate side toward the surface of the film (in the direction from the lower electrode side toward the upper electrode side) is put into practical use, it is necessary to drive the structure by (1) setting the upper electrode to a ground potential and the lower electrode to a positive potential as an address electrode, or by (2) setting the lower electrode to a ground potential and the upper electrode to a negative potential as an address electrode in order to align the orientation of the electric field with that of the spontaneous polarization.
In the case of (1) above, it is necessary to make the lower electrode on the substrate side to individual electrodes, which gives rise to a problem that the manufacturing process is complicated. In the case of (2) above, on the other hand, a negative voltage drive IC is required, which is larger than a positive voltage drive IC. This poses problems that the overall size of a device having the negative voltage drive IC is increased, the number of IC elements producible from one wafer is reduced, and the device cont is increased since the negative voltage drive IC is more expensive than the positive voltage drive IC.
Consequently, it is desirable that the lower electrode is not separated into individual electrodes and driven by a positive voltage drive IC by reversing the spontaneous polarization of a piezoelectric film. As for the reverse polarization method for reversing the spontaneous polarization of a piezoelectric film, a method that applies a voltage between the upper and lower electrodes sandwiching the piezoelectric film may be used, as in conventional poling. Further, it is desirable that the spontaneous polarization of piezoelectric films of a plurality of piezoelectric elements of a piezoelectric element structure is reversed at a time. In this case, a voltage is applied between the electrodes of each piezoelectric film and the spontaneous polarization of piezoelectric films of a multiple elements is reversed at a time.
An experiment conducted by the inventor of the present invention, however, showed the following problem. That is, when the simultaneous polarization reversal process described above was performed on a piezoelectric element structure under a temperature of around 100° C., the element defect rate due to film destruction at the time of voltage application amounted to as high as more than 30%, causing a problem that the piezoelectric element structure could not be put into practical use.
Generally, it is known that the film destruction at the time of voltage application is triggered by a portion of a piezoelectric film having a relatively low electric resistance, such as compositionally or structurally defective portion, or a portion of the piezoelectric film having a shape that is likely to attract charges, such as a defective surface area or a pore area. A continuous voltage application to the piezoelectric film causes heat to be generated in the charge concentration area in the trigger portion and the temperature of the piezoelectric film is locally increased. It is thought that a rapid decrease in the resistance in the locally heated area leads to charge concentration and film destruction.
When poling a piezoelectric element structure, if a voltage is applied between a solid lower electrode common to a plurality of elements and an upper electrode provided with respect to each element simultaneously, the total area of the electrode is increased and the destruction due to charge concentration is likely to occur, since the total amount of charges is increased as the total electrode area is increased. Consequently, it has been difficult to uniformly reverse the polarization of a piezoelectric element structure, resulting in a high defect rate and the difficulty in putting the piezoelectric element structure into practical use.
The problem of film destruction at the time of poling is significant for Pb system piezoelectric bodies having many composition defects of Pb dropouts at the time of forming. In addition, the problem is also significant for doped PZT systems or relaxor system Pb doped PZT having a low Curie point. Further, a piezoelectric film of less than 100 μm is likely to be destroyed by voltage application in comparison with a bulk piezoelectric body.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide a piezoelectric film polarization method for orienting the spontaneous polarization of a piezoelectric film in a predetermined direction without destruction. It is a further object of the present invention to provide a method for manufacturing a practical piezoelectric element structure with a piezoelectric film having spontaneous polarization oriented in a direction from an upper electrode toward a lower electrode.